The present invention relates to a method for manufacturing a spark plug which has an electric resistor interposed between a center electrode and a stem and causes a spark discharge between the center electrode and a ground electrode.
Unexamined Japanese patent publication No. 11-251033 discloses a conventional manufacturing method for a spark plug.
According to this manufacturing method, an electric resistive powder material chiefly containing a glass component is stuffed in an inside hollow space of an insulator. A plurality of insulators each accommodating the resistive powder material are heated together in a furnace. After being thermally processed, these insulators are conveyed out of the furnace. Next, the stem is depressed into each insulator under a lower-temperature atmosphere. Then, a metallic housing equipped with a ground electrode is securely assembled with the insulator by caulking.
Heating the insulator in the furnace is to sinter the electric resistive powder material to form an electric resistor located between the center electrode and the stem in the insulator. The electric resistance value of thus sintered electric resistor is dependent on a component ratio of the resistive powder material and also dependent on a sintering temperature in the furnace.
Through numerous experiences the inventor has found the fact that suppressing the dispersion of resistance values of electric resistors accommodated in a plurality of insulators thermally processed together becomes difficult when the resistance value of an electric resistor exceeds 3 kxcexa9, even if the material component ratio and the sintering temperature are carefully controlled.
As a result of a detailed inspection, the inventor has reached a conclusion that a cooling rate of each insulator gives a great influence on a resulting resistance value of the sintered electric resistor. This is similar to the phenomenon that mechanical properties (i.e., hardness and tensile strength) of a carbon steel vary depending on the cooling rate.
Especially, when a plurality of insulators are conveyed out of a furnace, some insulators positioned close to the entrance of the furnace are cooled early by the air coming into the furnace. Such a local cooling by the air coming into the furnace is believed to cause a large dispersion of resistance values among the sintered electric resistors.
Accordingly, the present invention has an object to provide a manufacturing method for a spark plug capable of suppressing the dispersion of electric resistance values among a plurality of insulators when thermally processed together.
In order to accomplish the above and other related objects, the present invention provides a first method for manufacturing a spark plug which has a center electrode and a ground electrode to cause a spark discharge and has an electric resistor interposed between the center electrode and a stem equipped with a terminal. The first manufacturing method comprises a step of stuffing an electric resistive powder material in an inner hollow space of each insulator into which the center electrode and the stem are installed, a step of heating a plurality of insulators in a furnace, a step of uniformly cooling the plurality of insulators when the plurality of insulators are conveyed out of the furnace, and a step of inserting the stem in the inner hollow space of each insulator.
The first manufacturing method makes it possible to substantially equalize a resistance value of the electric resistor in an insulator located close to the entrance with a resistance value of the electric resistor in another insulator located far from the entrance. Accordingly, the first manufacturing method effectively suppresses the dispersion of electric resistance values among a plurality of spark plugs. The first manufacturing method not only improves the yield of the spark plug but also reduces the manufacturing cost for the spark plug.
The present invention provides a second method for manufacturing a spark plug which has a center electrode and a ground electrode to cause a spark discharge and has an electric resistor interposed between the center electrode and a stem equipped with a terminal. The second manufacturing method comprises a step of stuffing an electric resistive powder material in an inner hollow space of each insulator into which the center electrode and the stem are installed, a step of mounting a plurality of insulators each accommodating the electric resistive powder material on a tray, a step of conveying the tray carrying the plurality of insulators into a furnace via an entrance of the furnace, a step of heating all of the plurality of insulators mounted on the tray in the furnace, a step of conveying the is, tray mounting the plurality of insulators thereon out of the furnace, and a step of inserting the stem in the inner hollow space of each insulator. The second method is characterized in that the tray has a windbreak positioned close to the entrance of the furnace when placed in the furnace for shielding the flow of air entering via the entrance.
The second manufacturing method makes it possible to prevent the insulator located close to the entrance of the furnace from being directly cooled by the air coming into the furnace when the tray mounting thermally processed insulators thereon is conveyed out of the furnace.
Accordingly, the second manufacturing method makes it possible to uniformly cool all of the insulators mounted on the tray when conveyed out of the furnace after finishing the sintering operation. The second manufacturing method makes it possible to substantially equalize a resistance value of the electric resistor in an insulator located close to the entrance with a resistance value of the electric resistor in another insulator located far from the entrance. Accordingly, the second manufacturing method effectively suppresses the dispersion of resistance values of the electric resistors accommodated in a plurality of insulators thermally processed together. The second manufacturing method not only improves the yield of the spark plug but also reduces the manufacturing cost for the spark plug.
According to a preferred embodiment of the second manufacturing method, the windbreak prevents a portion corresponding to the electric resistor from being directly cooled by the air.
This makes it possible to surely reduce the dispersion of resistance values of the electric resistors accommodated in a plurality of insulators thermally processed together.
The present invention provides a third method for manufacturing a spark plug which has a center electrode and a ground electrode to cause a spark discharge and has an electric resistor interposed between the center electrode and a stem equipped with a terminal. The third manufacturing method comprises a step of stuffing an electric resistive powder material in an inner hollow space of each insulator into which the center electrode and the stem are installed, a step of placing a plurality of insulators each accommodating the electric resistive powder material in receiving holes of a tray, a step of conveying the tray carrying the plurality of insulators into a furnace, a step of heating all of the plurality of insulators mounted on the tray in the furnace, a step of conveying the tray mounting the plurality of insulators thereon out of the furnace, and a step of inserting the stem in the inner hollow space of each insulator. The third manufacturing method is characterized in that each receiving hole of the tray is so deep that the portion corresponding to the electric resistor can be positioned or concealed in the receiving hole.
The third manufacturing method makes it possible to prevent the insulator located close to the entrance of the furnace from being directly cooled by the air coming into the furnace when the tray mounting thermally processed insulators thereon is conveyed out of the furnace.
Accordingly, the third manufacturing method makes it possible to uniformly cool all of the insulators mounted on the tray when conveyed out of the furnace after finishing the sintering operation. The third manufacturing method makes it possible to substantially equalize a resistance value of the electric resistor in an insulator located close to the entrance with a resistance value of the electric resistor in another insulator located far from the entrance. Accordingly, the third manufacturing method effectively suppresses the dispersion of resistance values of the electric resistors accommodated in a plurality of insulators thermally processed together. The third manufacturing method not only improves the yield of the spark plug but also reduces the manufacturing cost for the spark plug.
The present invention provides a fourth method for manufacturing a spark plug which has a center electrode and a ground electrode to cause a spark discharge and has an electric resistor interposed between the center electrode and a stem equipped with a terminal. The fourth manufacturing method comprises a step of stuffing an electric resistive powder material in an inner hollow space of each insulator into which the center electrode and the stem are installed, a step of mounting a plurality of insulators each accommodating the electric resistive powder material on a tray, a step of conveying the tray carrying the plurality of insulators into a furnace via an entrance of the furnace, a step of heating all of the plurality of insulators mounted on the tray in the furnace, a step of conveying the tray mounting the plurality of insulators thereon out of the furnace, and a step of inserting the stem in the inner hollow space of each insulator. The fourth manufacturing method is characterized in that the tray has a configuration for enlarging a cooling rate of an insulator located far from the entrance of the furnace compared with a cooling rate of an insulator located close to the entrance of the furnace.
The fourth manufacturing method makes it possible to positively cool the insulator located far from the entrance of the furnace. Accordingly, the fourth manufacturing method makes it possible to substantially equalize the cooling rates of respective insulators mounted on the tray so that all of the insulators mounted on the tray can be uniformly cooled when conveyed out of the furnace after finishing the sintering operation.
The forth manufacturing method makes it possible to substantially equalize a resistance value of the electric resistor in an insulator located close to the entrance with a resistance value of the electric resistor in another insulator located far from the entrance. Accordingly, the fourth manufacturing method effectively suppresses the dispersion of resistance values of the electric resistors accommodated in a plurality of insulators thermally processed together.
According to a preferred embodiment of the fourth manufacturing method, the tray has receiving holes for receiving respective insulators. A depth of a receiving hole provided close to the entrance of the furnace is deeper than a depth of a receiving hole provided far from the entrance of the furnace.
This makes it possible to surely reduce the dispersion of resistance values of the electric resistors accommodated in a plurality of insulators thermally processed together.
The present invention brings great effects when the electric resistor is equal to or larger than 3 kxcexa9.